Study of the impact of the polarity and spectrum of the THz pulse on ion emission: application to the atom probe

The Atom Probe Tomography (APT), the laboratory's flagship technique, enables the 3D reconstruction of the chemical distribution of a nano-object, with spatial resolution close to the atomic scale, using field evaporation of ions from the surface. In laser-assisted APT, commonly used to study metals, insulators and semiconductors, evaporation is triggered by an ultrafast laser pulse in the near ultraviolet (UV) range [1-2]. The energy of the UV pulses is absorbed by the sample and its heating degrades the instrument's performance in terms of spatial and chemical resolution.

Remarkable progress has been made using sources in the terahertz (THz) range, which has enabled the analysis of metallic materials as well as ceramics and oxides [3-5]. In addition, it has been shown that the emission process assisted by THz pulses minimizes heating effects. When using single-cycle THz pulses, an important role is played by their polarity (or sign) and spectrum. With positive pulses, the thermal effects are reduced in the case of metals, but are clearly visible in the case of band-gap materials. On the other hand, all materials can be analyzed with negative THz pulses and the performances are similar to that of UV lasers or degraded, depending on the spectrum of the THz pulse used.

In this context, a PhD thesis is being proposed to study the evaporation processes involved in the use of THz pulses as a function of their polarity and spectrum. To this end, it will be necessary to:

- Analyze materials with different band gaps in SAT-THz, by varying the polarity and spectrum of the pulse.

- Develop the generation bench in order to spectrally control the THz pulse generated.

- Couple the results of the analyses with numerical simulations of the trajectories of the ions and electrons emitted by the THz pulse.

Contact:

Interested candidates should send the following documents:

- Detailed CV with academic background.

- Letter of motivation

- Transcripts and rankings from Master 1 and Master 2.

Contact : angela.vella@univ-rouen.fr